Ultralong 250 km remote sensor system based on a fiber loop mirror interrogated by an optical time-domain reflectometer

A 253?km ultralong remote displacement sensor system based on a fiber loop mirror interrogated by a commercial optical time-domain reflectometer is proposed and experimentally demonstrated. The use of a fiber loop mirror increases the signal-to-noise ratio, allowing the system to interrogate sensors placed 253?km away from the monitoring system without using any optical amplification. The displacement sensor was based on a long period grating spliced inside of the loop mirror, which modifies the mirror reflectivity accordingly to the applied displacement.